Kubernetes auth method

The kubernetes auth method can be used to authenticate with Stronghold using a Kubernetes Service Account Token. This method of authentication makes it easy to introduce an Stronghold token into a Kubernetes Pod.

You can also use a Kubernetes Service Account Token to log in via JWT auth. See the section on How to work with short-lived Kubernetes tokens for a summary of why you might want to use JWT auth instead and how it compares to Kubernetes auth.

Note: If you are upgrading to Kubernetes v1.21+, ensure the config option disable_iss_validation is set to true. Assuming the default mount path, you can check with d8 stronghold read -field disable_iss_validation auth/kubernetes/config. See Kubernetes 1.21 below for more details.

Authentication

Via the CLI

The default path is /kubernetes. If this auth method was enabled at a different path, specify -path=/my-path in the CLI.

d8 stronghold write auth/kubernetes/login role=demo jwt=...

Via the API

The default endpoint is auth/kubernetes/login. If this auth method was enabled at a different path, use that value instead of kubernetes.

$ curl \
    --request POST \
    --data '{"jwt": "<your service account jwt>", "role": "demo"}' \
    http://127.0.0.1:8200/v1/auth/kubernetes/login

The response will contain a token at auth.client_token:

{
  "auth": {
    "client_token": "38fe9691-e623-7238-f618-c94d4e7bc674",
    "accessor": "78e87a38-84ed-2692-538f-ca8b9f400ab3",
    "policies": ["default"],
    "metadata": {
      "role": "demo",
      "service_account_name": "myapp",
      "service_account_namespace": "default",
      "service_account_secret_name": "myapp-token-pd21c",
      "service_account_uid": "aa9aa8ff-98d0-11e7-9bb7-0800276d99bf"
    },
    "lease_duration": 2764800,
    "renewable": true
  }
}

Configuration

Auth methods must be configured in advance before users or machines can authenticate. These steps are usually completed by an operator or configuration management tool.

  1. Enable the Kubernetes auth method:
  d8 stronghold auth enable kubernetes
  1. Use the /config endpoint to configure Stronghold to talk to Kubernetes. Use kubectl cluster-info to validate the Kubernetes host address and TCP port. For the list of available configuration options, please see the API documentation.
  d8 stronghold write auth/kubernetes/config \
      token_reviewer_jwt="<your reviewer service account JWT>" \
      kubernetes_host=https://192.168.99.100:<your TCP port or blank for 443> \
      kubernetes_ca_cert=@ca.crt

Note: The pattern Stronghold uses to authenticate Pods depends on sharing the JWT token over the network. Given the security model of Stronghold, this is allowable because Stronghold is part of the trusted compute base. In general, Kubernetes applications should not share this JWT with other applications, as it allows API calls to be made on behalf of the Pod and can result in unintended access being granted to 3rd parties.

  1. Create a named role:
  d8 stronghold write auth/kubernetes/role/demo \
      bound_service_account_names=myapp \
      bound_service_account_namespaces=default \
      policies=default \
      ttl=1h

This role authorizes the “myapp” service account in the default namespace and it gives it the default policy.

For the complete list of configuration options, please see the API documentation.

Kubernetes 1.21

Starting in version 1.21, the Kubernetes BoundServiceAccountTokenVolume feature defaults to enabled. This changes the JWT token mounted into containers by default in two ways that are important for Kubernetes auth:

  • It has an expiry time and is bound to the lifetime of the pod and service account.
  • The value of the JWT’s "iss" claim depends on the cluster’s configuration.

The changes to token lifetime are important when configuring the token_reviewer_jwt option. If a short-lived token is used, Kubernetes will revoke it as soon as the pod or service account are deleted, or if the expiry time passes, and Stronghold will no longer be able to use the TokenReview API. See How to work with short-lived Kubernetes tokens below for details on handling this change.

In response to the issuer changes, Kubernetes auth has been updated to not validate the issuer by default. The Kubernetes API does the same validation when reviewing tokens, so enabling issuer validation on the Stronghold side is duplicated work. Without disabling Stronghold’s issuer validation, it is not possible for a single Kubernetes auth configuration to work for default mounted pod tokens with both Kubernetes 1.20 and 1.21.. See Discovering the service account issuer below for guidance if you wish to enable issuer validation in Stronghold.

How to work with short-lived kubernetes tokens

There are a few different ways to configure auth for Kubernetes pods when default mounted pod tokens are short-lived, each with their own tradeoffs. This table summarizes the options, each of which is explained in more detail below.

Option All tokens are short-lived Can revoke tokens early Other considerations
Use local token as reviewer JWT Yes Yes Requires Stronghold to be deployed on the Kubernetes cluster
Use client JWT as reviewer JWT Yes Yes Operational overhead
Use long-lived token as reviewer JWT No Yes  
Use JWT auth instead Yes No  

Note: By default, Kubernetes currently extends the lifetime of admission injected service account tokens to a year to help smooth the transition to short-lived tokens. If you would like to disable this, set [–service-account-extend-token-expiration=false][k8s-extended-tokens] for kube-apiserver or specify your own serviceAccountToken volume mount. See here for an example.

Use local service account token as the reviewer JWT

When running Stronghold in a Kubernetes pod the recommended option is to use the pod’s local service account token. Stronghold will periodically re-read the file to support short-lived tokens. To use the local token and CA certificate, omit token_reviewer_jwt and kubernetes_ca_cert when configuring the auth method. Stronghold will attempt to load them from token and ca.crt respectively inside the default mount folder /var/run/secrets/kubernetes.io/serviceaccount/.

d8 stronghold write auth/kubernetes/config \
    kubernetes_host=https://$KUBERNETES_SERVICE_HOST:$KUBERNETES_SERVICE_PORT

Use the Stronghold client’s JWT as the reviewer JWT

When configuring Kubernetes auth, you can omit the token_reviewer_jwt, and Stronghold will use the Stronghold client’s JWT as its own auth token when communicating with the Kubernetes TokenReview API. If Stronghold is running in Kubernetes, you also need to set disable_local_ca_jwt=true.

This means Stronghold does not store any JWTs and allows you to use short-lived tokens everywhere but adds some operational overhead to maintain the cluster role bindings on the set of service accounts you want to be able to authenticate with Stronghold. Each client of Stronghold would need the system:auth-delegator ClusterRole:

kubectl create clusterrolebinding myapp-client-auth-delegator \
  --clusterrole=system:auth-delegator \
  --group=group1 \
  --serviceaccount=default:svcaccount1 \
  ...

Continue using long-lived tokens

You can create a long-lived secret using the instructions here and use that as the token_reviewer_jwt. In this example, the myapp service account would need the system:auth-delegator ClusterRole:

kubectl apply -f - <<EOF
apiVersion: v1
kind: Secret
metadata:
  name: myapp-k8s-auth-secret
  annotations:
    kubernetes.io/service-account.name: myapp
type: kubernetes.io/service-account-token
EOF

Using this maintains previous workflows but does not benefit from the improved security posture of short-lived tokens.

Use JWT auth

Kubernetes auth is specialized to use Kubernetes’ TokenReview API. However, the JWT tokens Kubernetes generates can also be verified using Kubernetes as an OIDC provider. The JWT auth method documentation has instructions for setting up JWT auth with Kubernetes as the OIDC provider.

This solution allows you to use short-lived tokens for all clients and removes the need for a reviewer JWT. However, the client tokens cannot be revoked before their TTL expires, so it is recommended to keep the TTL short with that limitation in mind.

Discovering the service account issuer

Note: disable_iss_validation and issuer are deprecated and the default for disable_iss_validation has changed to true for new Kubernetes auth mounts. The following section only applies if you have set disable_iss_validation=false , but disable_iss_validation=true is the new recommended value for all versions of Stronghold.

Kubernetes 1.21+ clusters may require setting the service account issuer to the same value as kube-apiserver’s --service-account-issuer flag. This is because the service account JWTs for these clusters may have an issuer specific to the cluster itself, instead of the old default of kubernetes/serviceaccount. If you are unable to check this value directly, you can run the following and look for the "iss" field to find the required value:

echo '{"apiVersion": "authentication.k8s.io/v1", "kind": "TokenRequest"}' \
  | kubectl create -f- --raw /api/v1/namespaces/default/serviceaccounts/default/token \
  | jq -r '.status.token' \
  | cut -d . -f2 \
  | base64 -d

Most clusters will also have that information available at the .well-known/openid-configuration endpoint:

kubectl get --raw /.well-known/openid-configuration | jq -r .issuer

This value is then used when configuring Kubernetes auth, e.g.:

d8 stronghold write auth/kubernetes/config \
  kubernetes_host="https://$KUBERNETES_SERVICE_HOST:$KUBERNETES_SERVICE_PORT" \
  issuer="\"test-aks-cluster-dns-d6cbb78e.hcp.uksouth.azmk8s.io\""

Configuring kubernetes

This auth method accesses the Kubernetes TokenReview API to validate the provided JWT is still valid. Kubernetes should be running with --service-account-lookup. This is defaulted to true from Kubernetes 1.7. Otherwise deleted tokens in Kubernetes will not be properly revoked and will be able to authenticate to this auth method.

Service Accounts used in this auth method will need to have access to the TokenReview API. If Kubernetes is configured to use RBAC roles, the Service Account should be granted permissions to access this API. The following example ClusterRoleBinding could be used to grant these permissions:

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: role-tokenreview-binding
  namespace: default
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: system:auth-delegator
subjects:
  - kind: ServiceAccount
    name: myapp-auth
    namespace: default

API

The Kubernetes Auth Plugin has a full HTTP API. Please see the API docs for more details.